1. Field
The present invention relates generally to photovoltaic energy conversion systems, and more specifically to unipolar, non-isolated photovoltaic energy conversion systems.
2. Background
Referring to FIG. 1, shown is a typical prior art photovoltaic (PV) system that depicts four inverters coupled between a distribution panel and four corresponding portions of a photovoltaic array. For simplicity, one array portion 102 and one corresponding inverter 106 are shown and three other array-inverter combinations are represented but not shown in detail. As shown, each photovoltaic array includes a plurality of strings that may be stacked up to one of a variety voltages (e.g., 1000 VDC).
As shown, each of the four inverters connects to the distribution panel that is coupled to a wye-delta transformer that may reside on the same skid as the inverters. In this conventional approach, the center star point reference on the primary side of the wye-delta transformer becomes the singular electrical system ground reference for all four inverters. In other words, each of the inverters derives its reference from the AC phases themselves, which are low impedance and symmetrically referenced around ground. The low impedance and symmetry are translated through each of the inverters so that a plus-minus even voltage split occurs around ground potential at the inputs to each inverter. For exemplary purposes, in this depicted system, the differential voltage across each inverter is 700 Volts so that an even split of around +350 and −350 Volts is at the inputs to the inverters.
Operating inverters in a bipolar (e.g., one input of the inverter 106 sits above ground and the other input sits below ground potential) approach has several advantages, but many photovoltaic modules have a polarity restriction that prevents integrators from utilizing a bipolar architecture. In particular, some thin film panels undergo a destructive degradory process when operated below ground potential. As a consequence, many panel manufacturers will not warranty their panels for use in bipolar architectures; thus it is known that this type system depicted in FIG. 1 needs to be modified so that the arrays operate entirely above ground potential (or entirely below ground potential for some photovoltaic panel types).
The depicted system in FIG. 1 is also a non-isolated system in the sense that the AC side of each inverter is not isolated from other inverters by a corresponding transformer. The depicted transformerless system is beneficial from the perspective that expensive and heavy isolation transformers installed for each inverter are avoided. But in the depicted transformerless system, it is not possible to convert the system to a unipolar mode of operation by simply placing a ground connection on the negative rail input of each inverter (or a ground connection to the positive rail), which can be done in systems that utilize transformer isolation techniques. It should be recognized that the wye-delta transformer depicted in FIG. 1 operates to convert the relatively low AC voltage at the outputs of the inverters to a high AC voltage for distribution—it does not function to isolate the inverters from each other.
As a consequence, to retain the benefits of multiple (in this example four) non-isolated inverters (e.g., which do not have four individual transformers or a transformer with four separate sets of secondary windings) an alternative approach to grounding one of the inverter input rails is required to operate the photovoltaic panels in a unipolar configuration.
As shown in FIG. 2, one way to place the array above ground potential (or below ground potential) is to place a power supply on the star point of the distribution transformer to elevate the secondary voltage with respect to ground by a DC common-mode voltage. For example, if the star point is elevated by 500 Volts above ground potential, the array would operate above ground potential when the inverters are processing power. This approach, however, requires a power source, which is costly and requires energy to provide continuing power. As a consequence, there is a need in the art for alternative approaches to converting the system depicted in FIG. 1 to a unipolar mode of operation.